I. Field of the Invention
This invention relates generally to tubular heat exchangers and, more particularly, to tube arrangements and supports used in heat exchanger tube bundles.
II. Description of Related Art
Tubular heat exchangers typically consist of a collection of pipes or tubes through which a hot fluid flows internally while a cooler fluid flows over the external surface of the tubes. Heat from the internal fluid transfers through conduction to the external surface of the tubes. This heat is then absorbed by the external fluid as it flows along the outer surface of the tubes, thus cooling the internal fluid. The external surface of the tubes acts as a heat transfer surface. Radial or longitudinal fins are often attached to the external surface of the tubes to increase the area of heat transfer surface and increase the tube's overall heat transfer rate.
Several tubes are combined in heat exchanger tube bundles and are typically enclosed in a shell. In the past, tube bundles have been constructed with tube supports or baffles located within the bundle to provide structural support and spacing for the tubes. The tubes must be firmly supported relatively to each other and to the outer shell, but the considerable length of the tubes often makes it impossible to support the tubes in each bundle from only the ends of the tubes. Several supports or baffles are typically located along the length of the bundle to provide the necessary support and spacing for the tubes.
Although these supports and baffles may often help increase the heat exchange rate of the tube bundle by directing fluid over the tube surfaces, these internal structures constrict fluid flow, increasing the pressure drop of the external fluid ("shellside fluid"), and adding to the costs expended pumping the shellside fluid through the bundle. Heat exchanger designers are generally as concerned with the shellside pressure drop as they are with the overall heat transfer rate of the tube bundle because the power required to move the fluid over the surface of the tubes is often a major operating expense. This pumping power expense is directly proportional to the pressure drop. Tube supports and baffles also significantly contribute to the costs of constructing and maintaining a tube bundle. A tube bundle designed without the use of internal tube supports or baffles improves the prior art by reducing shellside pressure drop and reducing the costs of constructing and maintaining a heat exchanger tube bundle.
As mentioned above, radial or longitudinal fins are often attached to the outer surface of the tubes to increase the area of heat transfer surface and increase the tubes' overall heat transfer rate. Although fins increase the heat transfer rate of the tubes, they take up a considerable amount of space in the tube bundle and reduce the number of tubes that can fit within a given tube bundle's cross sectional area. Tubes with very short fins ("low fins") can be used to maintain a small tube-to-tube distance ("tube pitch") and increase the number of tubes that will fit within a given shell size, but these low fin tubes are very expensive. Low fin tubing has fins embedded into the tube surface in a helical pattern. These fins cannot be used in tube bundles where low shellside pressure drop is required because low fin tubes with longitudinal fins are not available. Longitudinal fins are used in tube bundles where low shellside pressure drop is required because the fin surfaces are parallel to the direction of external fluid flow. A common method of constructing finned tubes is by welding or soldering fins onto bare tubes. Tubes that are constructed with welded-on or soldered-on fins cannot be finned with very low fins because of the difficulty of forming low fins and attaching the fins to the tubes.
Heat exchanger designers using finned tubes are therefore continuously plagued with the problem of being forced to reduce the total number of tubes installed in a given sized shell in order to take advantage of the increased heat transfer surface area provided by the addition of the fins.
By mixing finned and bare (non-finned) tubes in a tube bundle, I have discovered that it is possible to economically construct a tube bundle with a number of welded-on or soldered-on fintubes and still maintain a small tube pitch, thereby adding heat transfer surface without reducing the number of tubes in the bundle. The use of longitudinal fins with no spacing between the fins and adjacent bare tubes additionally allows the longitudinal fins to act as supporting means along the length of the tubes. Mixing finned and bare tubes also significantly reduces the cost of producing the heat exchanger tube bundle because of the reduced manufacturing costs of plain tubes.
Although some past heat exchanger designs have used a mixture of finned and bare tubes, none have disclosed the use of the finned tubes to act as supporting means. Two examples of prior patents that disclose the use of a mixture of finned and bare tubes are U.S. Pat. No. 3,111,168, issued to Andre Huet in 1963, and U.S. Pat. No. 2,828,723, issued to Avy L. Miller in 1958. Although both issued patents disclose the use of a combination of finned and bare tubes, the inventions are limited to the use of the finned tubes as baffles to direct the flow of shellside fluid and increase heat exchanger performance. For example, in the Huet patent, the finned tubes are positioned so that the fins direct fluid flow and force it to bathe the surface of the plain tubes. In the Miller patent, the unfinned tubes are positioned to act as baffles, directing the flow of hot flue gasses in a water heater to bathe the surface of the finned tubes.
No prior art designs have taken advantage of the use of a mixture of finned and bare tubes in which longitudinal fins provide support for the bare tubes substantially along the entire length of the tubes. Such a design with a mixture of finned and bare tubes reduces the costs of producing the tube bundle and eliminates the need for internal tube supports or baffles that increase shellside pressure drop.